Coronal activity among open cluster stars

Abstract

Focusing on ROSAT results for clusters in the $\sim 20-600$ Myr range, I first summarize our current understanding of the X-ray activity - rotation - age relationship. Then, the problem of the Hyades K and M dwarfs binaries is addressed: 1. most K and M-type binaries in wide systems are X-ray brighter than single stars; 2. binaries seem to fit into the same activity - rotation relationship as single stars. Points 1. and 2. suggest that the distributions of rotations of single and binary stars should also show a dicothomy, but the few available rotational data do not support the existence of such a dicothomy. Rotational periods for a larger sample of binary and single stars should be acquired before any conclusion is drawn. Finally, I discuss the topic whether the activity-age dependence is unique, as commonly thought. Whereas the comparison of Praesepe to the Hyades might imply that this is not the case, the X-ray activity of a sample of Hyades-aged field stars instead supports the common thinking.